(a) Field of the Invention
The present invention relates to a VDSL (Very high bit-rate Digital Subscriber Line) system. More specifically, the present invention relates to a zipper type VDSL system providing efficient cyclic extension.
(b) Description of the Related Art
Generally, in the zipper system, a cyclic prefix (hereinafter referred to as “CP”) and a cyclic suffix (hereinafter referred to as “CS”) are added to a block called discrete multi-tone (hereinafter referred to as “DMT”).
The CP is inserted in order to prevent interference between symbols and between subchannels, and the CS is to maintain orthogonality between upstream and downstream sub-carriers. Thus insertion of both CP and CS into the DMT secures prevention of near-end cross talk (hereinafter referred to as “NEXT”).
Such a zipper system may be regarded as an extended form of the DMT system in two aspects as follows: (1) the subchannels are dynamically allocated adequately to a required bit rate for upstream and downstream, which allows the system to share the upstream and downstream with other systems such as ADSL (Asymmetric Digital Subscriber Line) in a single binder, making it easier to use the system; and (2) as well as the CP, the CS is added to the end of the signal so as to prevent NEXT.
As shown in FIG. 1, the NEXT signal is received in a short time from an adjacent network terminal but a desired signal is delayed by Δ, which depends on the length of the channel.
If not in ideal circumstances, upstream and downstream signals using distinct sub-carriers may affect each other because they fail to maintain orthogonality with the NEXT signal due to the channel delay Δ.
For that reason, the zipper system adds the CS to the end of the DMT frame in order to maintain orthogonality with the NEXT signal.
Contrary to CP, the CS copies the leading part of a desired signal and it is added to the end of the signal. Adding the CS allows the signal to maintain orthogonality with the NEXT signal and thereby makes it possible to fully restore a transmission signal from a received one at the reception terminal.
The latency generally varies depending on the length of the channel, and thus, in the synchronous mode, the channel of the longest latency in the single binder determines the length of the CS.
Unlike the synchronous mode in which all frames in the binder to be transmitted are synchronized with one another, the asynchronous mode causes a network terminal to transmit signals at any time and fails to maintain orthogonality between upstream and downstream.
To overcome the problem, in the asynchronous mode, a pulse shaping function at the transmission terminal and a windowing function at the reception terminal are effected to minimize spreading of signals out of given upstream and downstream bands.
The windowing function also reduces the sidelobe of radio frequency interference (RFI) introduced in the system.
Compared to the synchronous mode, the asynchronous zipper system is more efficient because the length of CS is adjustable depending on the latency of the individual channels.
FIG. 2 is a diagram explaining pulse shaping and windowing functions for a DMT symbol in the conventional zipper system.
As shown in FIG. 2, the asynchronous zipper system mainly uses a raised-cosine window in order to maintain orthogonality of signals.
The term “latency” as used herein indicates a maximum time required for one bit to be fed into the transmission terminal and be output at the reception terminal. In the zipper system, latency is given by the following equation:τZipper=Δ+(2(2N+NCP)+NCS)/fs
In the equation, Δ represents the latency caused by the channel, N the number of subchannels, NCP the length of the CP, NCS the length of the CS, and fs the sampling frequency.
The efficiency of the zipper system is also given by:       δ    Zipper    =            2      ⁢      N                      2        ⁢        N            +              N        CP            +              N        CS            
Table 1 shows the efficiency and latency of a zipper system with the length of the channel being 1,500 m, NCP 60 samples, NCS 150 samples, and fs 20 MHz.
TABLE 1Efficiency and latency of zipper type VDSL system.No. of subchannelsEfficiencyLatency25670.9% 72 μs102490.7%226 μs409697.5%840 μs
As is apparent from Table 1, the efficiency increases with a larger number of subchannels and the latency is much smaller compared to that of the (?) type VDSL system such as SDMT.
FIG. 3 is a schematic block diagram of the conventional zipper type VDSL system.
As shown in FIG. 3, the conventional zipper type VDSL system comprises: a transmitter 10 including an Inverse Fast Fourier Transform (hereinafter referred to as “IFFT”) unit 12 for performing an IFFT on input signals, and a cyclic extension adder 14 for adding the CP and CS to the signals transformed by the IFFT unit 12; a receiver 20 including a cyclic extension remover 22 for removing the CP and CS from the signals output from the transmitter 10, and an FFT unit 24 for performing an FFT on the signals output from the cyclic extension remover 22; and a transmission channel 30 for exchanging the signals between the transmitter 10 and the receiver 20.
Various components other than the above-mentioned ones may be included in the transmitter 10 and the receiver 20 of the conventional zipper type VDSL system and will not be described in further detail.
Also, a description will be given on the assumption that the signal symbol processed at the cyclic extension adder 14 and the cyclic extension remover 22 is composed of 2N effective data, NCP CP's, and NCS CS's.
The CP is used to prevent the previous symbol transmitted on a channel with a memory from being damaged by the current symbol and, as illustrated in FIG. 4, copies the ending part of the symbol in the time domain to maintain the circular form of the effective data.
In this regard, a part of the symbol to be copied into the CP is later than its inserting position and thus the previous input signal must be stored before an input of the next signal.
The CS is used to remove the NEXT, and contrary to the CP, copies the leading part of the symbol.
After insertion of the cyclic extension at the cyclic extension adder 14, the received signal is expressed by:                     x        ~            l        ⁢          (              n        ~            )        =            1                        2          ⁢          N                      ⁢                  ∑                  k          =          0                                      2            ⁢            N                    -          1                    ⁢                           ⁢                                    X            l                    ⁢                      (            k            )                          ⁢                  ⅇ                                    j2π              ⁢                                                           ⁢              k              ⁢                              n                ~                                                    2              ⁢              N                                          
Here, ñ=−NCP, −NCP+1, . . . , 2N+NCS−1 and Xl is transmitted data symbol.
The signal received through the transmission channel 30 generally contains phase jitter and timing offset added at the sampler and is thus given by:                     y        ~            l        ⁢          (              n        ~            )        =                    1                              2            ⁢            N                              ⁢                        ∑                      k            =            0                                              2              ⁢              N                        -            1                          ⁢                                   ⁢                                            X              l                        ⁢                          (              k              )                                ⁢                                    H              l                        ⁢                          (              k              )                                ⁢                      ⅇ                                          j2π                ⁢                                                                   ⁢                                  k                  ⁡                                      (                                                                  n                        +                        δ                                            _                                        )                                                                              2                ⁢                N                                                          +                            w          ~                l            ⁢              (                  n          ~                )            
Here, δ is a normalized timing offset and HI(k) is the I'th channel frequency response and {tilde over (w)}l is the additive noise.
Due to interference between adjacent symbols while passing through the transmission channel 30, the signal distorted by the channel is received at the cyclic extension remover 22 of the receiver 20.
The distorted CP's signal is removed at the cyclic extension remover 22 to extract 2N effective data not affected by the interference between the symbols, and then demodulated at the FFT unit 24. This demodulated signal is given by:                                                                                                               y                    ~                                    l                                ⁡                                  (                                      n                    ~                                    )                                            =                                                                    1                                                                  2                        ⁢                        N                                                                              ⁢                                                            ∑                                              k                        =                        0                                                                                              2                          ⁢                          N                                                -                        1                                                              ⁢                                                                                   ⁢                                                                                            X                          l                                                ⁡                                                  (                          k                          )                                                                    ⁢                                                                        H                          l                                                ⁡                                                  (                          k                          )                                                                    ⁢                                              ⅇ                                                                              j2π                            ⁢                                                                                                                   ⁢                                                          k                              ⁡                                                              (                                                                  n                                  +                                  δ                                                                )                                                                                                                                          2                            ⁢                            N                                                                                                                                              +                                                                            w                      ~                                        l                                    ⁡                                      (                    n                    )                                                                                                                                                                                Y                    ~                                    l                                ⁡                                  (                  k                  )                                            =                                                1                                                            2                      ⁢                      N                                                                      ⁢                                                      ∑                                          k                      =                      0                                                                                      2                        ⁢                        N                                            -                      1                                                        ⁢                                                                           ⁢                                                                                                              y                          ~                                                l                                            ⁡                                              (                        n                        )                                                              ⁢                                          ⅇ                                                                                                    -                            j2π                                                    ⁢                                                                                                           ⁢                          nk                                                                          2                          ⁢                          N                                                                                                                                                                                            =                                                                                          X                      l                                        ⁡                                          (                      k                      )                                                        ⁢                                                            H                      l                                        ⁡                                          (                      k                      )                                                        ⁢                                      ⅇ                                                                  j2πδ                        ⁢                                                                                                   ⁢                        k                                                                    2                        ⁢                        N                                                                                            +                                                                            W                      ~                                        l                                    ⁡                                      (                    k                    )                                                                                                          (        1        )            
Here, n=0, 1, 2, . . . , 2N−1, and k=0, 1, 2, . . . , 2N−1.
FIG. 5 is a block diagram of a CP adding unit in the cyclic extension adder 14 of the conventional zipper type VDSL system.
As shown in FIG. 5, the CP adding unit of the cyclic extension adder 14 includes 2N buffers 141, 143 and 145 for delaying the data output from the IFFT unit 12 by 2N, i.e., the number of effective data.
If the IFFT processed data from the IFFT unit 12 are serially fed into the cyclic extension adder 14, a part of the data to be copied into the CP is later than its inserting position and thus the previous input data must be stored before an input of this data.
The number of data to be stored is 2N, and 2N buffers are required to store the data.
The buffers 141, 143 and 145 are under the control of a logical product circuit 147 for performing a logical product determination on clock and enable signals.
It is multiplexer 149 that adds the CP's to the output data of the IFFT unit 12.
Data output from the IFFT unit 12 and directly fed into the 0'th pin of the multiplexer 149 are earlier by one DMT symbol than those delayed via the buffers 141, 143 and 145 and fed into the I'th pin. So, the CP's can be inserted into the data by controlling the output of the multiplexer 149.